The present invention relates to a method of detecting a weak point in an exposure mask, and more particularly, to a method of detecting a weak point using a NILS (Normalized Image Log Slope).
Generally, an exposure mask (reticle) is required in addition to an exposure apparatus and a photoresist to form a pattern used for a photolithography process to produce a semiconductor. The exposure mask is an original plate which is used to repetitively form a pattern on a semiconductor substrate. It is made of a formed quartz plate of a chrome pattern having four or five times the size of the target pattern according to the reduction projection rate. The pattern on this exposure mask should have the same critical dimension for the same layout pattern. That is, the accuracy of a pattern becomes an important factor in the fabrication of the exposure mask. Recently, the demand on accuracy has increased more and more as the line width of the semiconductor device has decreased.
As the pattern gradually becomes minute due to a high integration of the semiconductor device, the patterns which are projected onto the semiconductor substrate through an exposure process can become distorted from the real image of an exposure mask pattern. Particularly, if a gap between adjacent patterns is short among exposure mask patterns, adjacent patterns affects each other and the pattern is distorted. This phenomenon is called as an optical proximity effect.
This optical proximity effect causes a weak point by not satisfying a specific specification that the critical dimension of layout has. At this time, the weak point is estimated by comparing a contour image obtained through a simulation with the specific specification that the critical dimension of has layout has.
In detail, the weak point means a point where the critical dimension of a layout and the critical dimension of a contour image do not coincide according to various process parameters. At this time, the region in which weak points are easily caused includes patterns in which the gap between nearby patterns is narrow, or patterns in which the width is narrow.
FIG. 1a is a contour image showing a weak point according to a related art, FIG. 1b is a light intensity profile showing a weak point according to the related art.
As shown in FIG. 1a, a part in which the critical dimension of the contour image does not satisfy a specific specification is determined by comparing the critical dimension of the layout with the critical dimension of the contour image. That is, like ‘A’ of FIG. 1a, it is the region in which the critical dimension of the gap between adjacent patterns is smaller than the critical dimension of the actual layout and such region is determined as a weak point.
As shown in FIG. 1b, the weak point is located by using the maximum value and minimum value of the light source intensity which penetrated the exposure mask. That is, like ‘B’ of FIG. 1b, as the intensity of the light source does not exceed a reference point (i.e., the horizontal line) and becomes lower than the reference point, it can become a bridged region with adjacent patterns, so that such a region is determined as a weak point.
However, there is a problem in that the method for detecting the above-described weak point may determine a weak point that is not an actual weak point. That is because that all the region has a intensity which is similar to the intensity changed by the process parameter which cannot be predicted according to the process variation of a mask is extracted as a weak point.
Accordingly, there is a problem in that a region which is not a weak point is modified such that another distortion of a wafer image is created.